As individual device dimensions become smaller in integrated circuitry, there is a continuing challenge to maintain sufficient conductance and current flow in spite of the smaller sizes. Conductive elemental metals provide reduced resistance in comparison to conductively doped silicon and metal silicides. However, elemental metals may be more problematic to deposit and/or etch in comparison to other materials. One low resistance material of interest is elemental tungsten. Such may be used as a conductive gate material in transistor gate constructions. However, elemental tungsten is difficult to etch when subtractively patterning a transistor gate, and may lead to tungsten contamination elsewhere on the substrate over which the tungsten is received.
Transistor gates are used in many different types of integrated circuitry, for example in logic circuitry and memory circuitry. Memory circuitry provides data storage for electronic systems, and may be volatile or non-volatile. Non-volatile memory may be characterized by data or programmed retention after the device is turned “off”. Volatile memory, on the other hand, is required to be repeatedly rewritten due to charge dissipation and is lost when the circuitry is turned “off”. Dynamic Random Access Memory (DRAM) is one type of volatile memory. Static random access memory and flash are two examples of non-volatile memory.
A flash memory is a type of EEPROM (electrically-erasable programmable read-only memory) that may be erased and reprogrammed in blocks. Many modern personal computers have BIOS stored on a flash memory chip. Such BIOS is sometimes called flash BIOS. Flash memory is also popular in wireless electronic devices as it enables manufacturers to support new communication protocols as they become standardized, and provides the ability to remotely upgrade the devices for enhanced features.
A typical flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. The cells are usually grouped into blocks. Each of the cells within a block may be electrically programmed by charging a floating gate. The charge may be removed from the floating gate by a block erase operation. Data is stored in a cell as charge in the floating gate.
NAND is a basic architecture of flash memory. A NAND cell unit comprises at least one select gate coupled in series to a serial combination of memory cells (with the serial combination being commonly referred to as a NAND string).
Flash memory incorporates charge storage structures into transistor gates, and incorporates control gate structures over the charge storage structures. The charge storage structures may be immediately over gate dielectric. The charge storage structures comprise material capable of storing/trapping charge and which is collectively referred to herein as floating gate material. The amount of charge stored in the charge storage structures determines a programming state. In contrast, standard field effect transistors (FETs) do not utilize charge storage structures as part of the transistors, but instead have a conductive gate directly over gate dielectric material.